BIOPROSTHETIC VALVE TISSUE VISCOELASTICITY - IMPLICATIONS ON ACCELERATED PULSE DUPLICATOR TESTING

Most of our knowledge of heart valve mechanics has been gained from lo w strain-rate studies much lower than physiologic levels. Using a high -speed materials testing system, we compared the low and high strain-r ate viscoelastic behavior of porcine aortic valve cusps at extension r ates of up to 40 mm/s. Circumferential and radial strips were stretche d and then held in their stretched configuration to measure their ''st ress-relaxation'' behavior. During low strain-rate stretching, only 6% of the initial stress dissipated or relaxed after 1 second, whereas 2 5% of the stress dissipated during high strain-rate stretching. This c onsiderable difference in stress relaxation suggests a rate-dependent viscoelastic behavior that has not been accounted for in valve design and may have important implications for accelerated pulse testing. Eve n though the valve cusp is loaded for only 0.4 seconds during each hea rtbeat, at least 15% of the stress may relax over that period. During accelerated pulse testing, however, sufficient time may not be availab le to allow the tissue fibers to relax back to their natural state bef ore the subsequent loading cycle, leading to a higher baseline preload . In addition, because valve tissue is not given sufficient time to re lax before the next cycle, pulse testing subjects the valves to lower- magnitude cyclic stresses than does physiologic loading. Because both the baseline preload and the magnitude of cyclic stresses may lead to early fatigue failure, accelerated wear testing may either overestimat e or underestimate valve durability. Clearly, the mechanism of stress- induced failure of biologic tissues must be elucidated before too much validity is placed on pulse duplicator studies.